The present invention relates to electrochemical measuring electrode devices for transcutaneous measurement of a blood parameter, such as the partial pressure of a blood gas. The transcutaneous measuring technique is well-known in the art. In accordance with the transcutaneous measuring technique, an electrode device for measuring the blood parameter in question is applied to a skin surface of a person in whom the blood parameter is to be measured. The electrode device is thermostated to a predetermined temperature, normally (when the blood parameter to be measured is, e.g., the partial pressure of a blood gas such as oxygen) a temperature above normal skin temperature so as to cause local hyperaemia in the skin surface in contact with the electrode.
Above certain minimum levels of perfusion in the skin area where the transcutaneous measurement is performed, parameters measured transcutaneously, e.g. blood gas partial pressures, reflect the corresponding arterial values which are the values normally used for clinical purposes. Below such minimum levels, the parameters measured transcutaneously can no longer be considered as reflecting the arterial values.
For this reason, it is important to monitor the local capillary blood flow concomittantly with the local transcutaneous measurement of a blood parameter. Furthermore, calculation methods have been suggested which convert the transcutaneously measured values into calculated values correlating to a higher degree with actual arterial values when, in addition to the perfusion, the metabolic oxygen consumption, the capillary temperature, and the skin diffusion gradient are also known or estimated.
It has been suggested, cf. e.g. Journal of Clinical Engineering, 6, No. 1, January/March 1981, pp 41-47 (Reference 1), Birth Defects, Original Article Series Vol. XV, No. 4, pp 167-182, 1979 (Reference 2), and Critical Care Medicine, October 1981, Vol. 9, No. 10, pp 736-741 (reference 3) to monitor the local capillary blood flow by measuring the power supplied to the transcutaneous electrochemical measuring devices to keep the devices at a constant temperature.
However, according to Reference 1, merely about 10-15% of the total sensor power is perfusion-dependent. Reference 2 suggests a device where the heat exchange with the surroundings is limited by means of a heat shell over the electrode device, the heat shell being circulated with water at electrode temperature, but reports that only about 30% of the heat transferred from the electrode to the skin is flow related. Reference 3 suggests a combined O.sub.2 /CO.sub.2 and flow sensor which is adapted to be mounted on the forearm of a test person. Apart from a first servo-controlled heater/thermistor arranged in heat-conductive contact with a first heater assembly and serving the above described purpose of causing local hyperemia in the skin surface in contact with the sensor, a second servo-controlled heater/thermistor is included and arranged in heat-conductive contact with a second heater assembly arranged on the outside of the sensor. The second servo-controlled heater/thermistor is adapted to maintain the temperature of the second heater assembly at a temperature of 0.5.degree. C. below the temperature of the first heater assembly. Thus, the second heater assembly which is not contacted with the skin surface merely serves thermal insulation purposes. When employed in conjunction with an occlusive system which is adapted to be arranged enclosing the entire forearm of a test person or patient and, furthermore, increases the insulation properties, the sensor is reported to be able to register a perfusion-dependent heat transfer to the skin of approximately 50% of the total sensor power.